Modulator



June 8, F. C; LUNNON ET AL MODLATOR Filed July 2l, 1952 Fav 20 faz/Rafaf m; j 5h Pam/rm ffm/Uff? of' am af? .faz/mf of asf.

FREDERICK c. LUNNON BfHARoLQJ. wAssELL ATTORNEY Patented June 8, 1937UNITED STATES PATENT OFFICE Hughes Wassell,

Chelmsford,

England, as-

signors to Radio Corporation of America, a corporation of DelawareApplication July kZ1, 1932, Serial No. 623,750

In Great Britain August 18, 1931 Claims.

so-called absorber keying is eifected in a radio or other high frequencycircuit arrangement of the so-called absorber type by keying meansarranged to alter the circuit conditions of a valve in such manner thatsaid Valve is connected in circuit as an absorber valve during spacingbut acts as an amplier valve during marking.

In the present specification the term absorber modulating arrangementsis employed to refer to an arrangement of the kind in which atransmitter, which is modulated, is kept under load during spacingperiods by means of one or more thermionic valves which are arranged toprovide a load' upon the source of anode potential when the transmitteris spacing,

The present invention provides an improvement in the invention disclosedin said United States application Serial No. 520,587, and the modifiedor improved arrangement is adapted for telephony,y as well as fortelegraph.

When a transmitter is employed, for example, in duplex telephony, it isoften desirable to suppress the transmitted carrier during intervals inspeech so as to avoid setting up parasitic noises inthe' receiver. Thepresent invention is particularly adapted for application to duplextelephone systems for this purpose though the said invention is not, ofcourse, limited to this particular application for this particularpurpose.

According to this invention carrier transmission is substantiallysuppressed during spacing, e. g. .during intervals in speech,'like 'in amodulated radio or other high frequency transmitter, said suppression ofcarrier being effected automatically by means operated in dependenceupon the modulation input, the modulating arrangement being such thatduring the said suppression periods there is utilized as an absorbervalve a valve which is utilized as an ampliiier valve during otherperiods. y

The invention is illustrated in Figs. 1 and 2 of the drawingaccompanying this specification, which drawing shows the nal magnierstage of a transmitter in accordance with thepresent invention. In thedrawing of the specication, parts corresponding to parts illustratedl inthe (Cl. Z50-17) drawing accompanying United States application Serial No. 520,587 are indicated with the same reference numerals as in the saiddrawing of Unitedv States application Serial No. 520,587. VThe earlierstages ofthe transmitter, of which the final stage is shown in thedrawing of the present application, may be as illustrated in the drawingaccom-y panying the specification of United States application SerialNo. 520,587. y

Referring to Figure 14 of the drawing, the final amplier valve V3 of atransmitter, whose self capacity may be neutralized 'as shown, may becoupled to the preceding stage I0 by means of a coupling condenser lcsand receives anode potential from a source l2. The stage l0 may be anamplifier of high frequency loscillations and may include a source ofhigh frequency oscillations. The grid circuit of the valve V3 includesthe grid electrode, the impedance Z1, the anodel to cathode impedance ofvalve MV and the impedance between the grid and anode tied together andthe cathode of GL in parallel as shown and extends yto a tapping pointPs upon a resistance Z. The circuit is completed by connecting theresistance Z to the cathode of Vs. The tapping point P; and 25resistance Z correspond to the similarly marked tapping point andresistance in the arrangement illustrated in the drawing accompanyingUnited States aplication Serial No; 520,587.

The impedances of the Valves MVand GL are 30 reversed as shown. Thepurpose of the valve GL is primarily to permit thenegative potential setupacross the resistance Z described more in detail hereinafter to beapplied to'the grid of the valve V3 by way of tapfPs on Z. This negativepotential is required during spacing periods or periods between voicefrequency modulations in order to set the valve V3 at such a point onits characteristic that it will take the required dead loss feed andthus act as an absorber valve. Obviously the said negative potentialrcannot reach the grid of valve V3 through the anode to cathode impedanceof the modulating valve MV because the said valve is connected the wrongway. The required negative potential might, of course, be supplied byWay of a high resistance shunting MV, butin this case the resistance4would have to be extremely high or it would disturb the modulatingcharacteristic of MV and mightcause distortion. Moreover, in thepresence of secondary emission from the grid of the valve Va duringspacing conditions, a positive potential might arise on the grid of V3sufficientlyk high' lto overcome the negative potential supplied bytheresistance'Z and thus cause excessivecurrent/to 4o y 1 reversedirection. l

flow in the anode circuit of V3. The use of the one-way rectifier orvalve GL overcomes the disadvantages which would arise from the use of aresistance at this point. The Valve GL serves the same purpose as a highresistance leak across the modulating valve MV without introducing thedangermentioned above.

The tube MV permitsthe impedance of the grid in the grid circuit of thevalve V3. The resistance Z may be shunted as shown by a loop circuitcomprising the anode to cathode impedance oi a valve K and a source ofanode potential B in series. Modulating potentials maybe applied to theinput electrodesof the valve -MV from, for exampleuthe secondary of amicrophone transformer -MT, whose primary Amay be energized froma`rr1icrophone M as shown. The microphone M mayalso apply potentials tothevinput terminalsof a speech amplifier and rectifier ATD.'Iherectifier ATD :may also Iinclude a so-called hang over or delaycircuitconsisting of resistance and capacity inknown arrangement toprovide orprloduce a predetermined time constant. The, cn'utput'of thedevice may be applied between the grid and the `cathode of the valve K1,which 'valve may, if desired, be replaced by a plurality of -valvesvinparallel. The grid and anode of the valve GL `are connected together asfs'hown and the `said valve is so arranged as in effect' to `constitutea rectifying diode allowing negative potential to reach the-grid of thevalve y:z-ybut yet maintaining a high resistance in the IIyrhe action ofYthe -whole arrangement is as follows: f" vw Y W Whilelspeech is takingplace rectied speech potentialsare impressed -on thegrid of vthe valveK1 lvia the time constant'device ATD, these` rectiiied Ypotentials beingof such value and the settin'g'f of the valve/Kibeing vsuoli that theapplication of thesaidyrectiiied potentials results in biasing the valveK1 to cut-off and thereby prevntingthes'ource of potential B fromYsending current thijough-the resistance Z.` 'I'he grids of the variousstages in the transmitter including thestage .Va showny are thereforeconnected to earth through ltheir respective tappings on the resistanceZ. The potential on` the grids are s'uchfthat'osvcillations aredeveloped in the preceding stages, andamplied and 'modulated atspeechv/frequenciesin the stage V3 andthe transmit-terZ-accordinglyfunctions in the normal manner.A rOn 4cessation-of speech, however, therecvoltageapplied ,tothefgridof the valve K1 will die'away at -a ratedepending upon the time constant of the hangover circuit incorporatedin" the` .time constant vdevice ATD. This time constant should bek sochosen that ther voltage onthefgrild ofthe u valve,-K1 is maintainedduring the intervals between words and phrases'but ceaseswhen anappreciable pause occurs.

When the rectified voltage is removed from the grid-pf valve K1' saidvalve becomes conductive, a! current riiow `occurs through the`resistance Z duefLto'the battery or other source B', and the variousstages Yinjthe amplifier will be either biased,.ljl y,potentialssupplied,v to said :stages ,from

the leads connected to points P0 and P2 on resistance Z, to cut-off anddraw no current or set to draw dead loss anode current thereby acting asabsorber valves in a manner similar to that described in United Statesapplication Serial No. 520,587 to maintain the load on the main sourceconstant to the `desired degree to improve regulation.

The signal modulated energy appearing in tank Vcircuit 20 may beutilized in any known manner.

It may be transferred to an antenna for radiation, to a load circuit, orto transmission lines.

The drawing illustrates in Figure 2 an arrangement wherein the presentinvention is applied to telegraphy.

The arrangement may be regarded as a modication of the arrangement shownin Figure l, only such parts being shown in Figure 2 as are necessary toillustrate the nature of the modication.

Referring to Fig. 2, the arrangement of Fig. 1 is modified :bysubstituting for -the lmicrophone M, microphone transformer MT, andassociated amplifier and rectifier and timeycontrol device ATD, vakeying relay or equivalent vdevice M', a resistance R, and a battery orother suitable source of potential B". 'I'he action of the arrangementin Fig. 2 is `similar tothe arrangement illustrated in Fig. lof thedrawing so far as the suppression of the carrier is concerned, and, as

will be apreciated, the operative diiferencels that the requirednegative biasing potential on the valve K is obtained, not from speechvoltages, but from the source of vpotential B". R is a. limitingresistance of high value. The arrangement illustrated in theIaccompanying drawing results in a. reduction of key clicksan importantimprovement more particularly where duplex and twoway telegraphytisin'question-andalso presents the-advantage that a'high speed oftelegraphy is possible since the Akeying contacts only have to handleminute currents rand `therefore the said key may be constituted by lahigh speed relay.

Having thus described our invention and the operationthereof, whatweclaim is:

1. `A modulationsystem of the type in which oscillations 'are producedand applied to a thermionic repeater valve in which they are modulatedand in which'the repeater isrenderedinoperative when no modulating-Apotentials are impressed `thereon,'and in which vpreceding stages ofthetransmitter system are biased to draw current when said repeater'isinoperative to thereby regulate the load `applied to .a common -sourceof 'potential'including a't'hermionic valve having its `anode "tocathode impedance connected in series'with vthe cathode 'to control gridimpedance of said 'repeater by way of a potentiometer, a

connected in parallel'with the anode to cathode impedance of4 saidthermionicvalve, a rectii'ying device having its output electrodesconnected by way of a source of Apotential in parallel to saidvpotentiometer, and a source 'of modulating potentials'connectedI to theinput electrodes of said rectifyingdeviceand tothe control grid andcathode of lsaid thermionic Y valve.

2. A'system'as recited in claim 1 in which said device, which isconductive in one direction only, comprises the internal impedance of athermionic valve 'which permits negative `potential to ,flow fromsaidsource connected with said `potentiometer ytothe con-troly grid-ofsaid repeater, `and i-n=\which a `time constant device is interposedbetween said source of modulating potentials and the input electrodes ofsaid rectifying device.

3. A device as recited in claim 1 in which a time control device isinterposed between said source of modulating potentials and the inputelectrodes of said rectifying device, and in which said potentiometer istapped at 'appropriate points to provide biasing potentials for thecontrol grids of valves in the transmitter preceding the repeater valverecited.

4. A modulating system of the type in which oscillations are producedand applied to a thermionic repeater in which they are modulated and inwhich the repeater is rendered inoperative when no modulating potentialsare impressed thereon including, a thermionic Valve having its anode tocathode impedance connected in series with the cathode to control gridimpedance of said repeater by way of a portion of the resistance of apotentiometer, a thermionic discharge device which is conductive in oneydirection only connected in` parallel with the anode to cathodeimpedance of said thermionic valve, a second thermionic device havingits output electrodes connected in parallel to the resistance of saidpotentiometer, and a source of modulating potentials connected to theinput electrodes of said last named thermionic device and to the controlgrid and cathode of said thermionic valve.

5. In a signalling system, a thermionic repeater of high frequencyoscillations having a control grid and a cathode connected in a circuitto be energized by said oscillations, a, lirst thermionic valve having acontrol electrode and having its internal impedance, in series with asource of direct current potential connected between the control gridand cathode of said repeater for normally biasing said repeater to apoint at which no current flows therein, a source of modulatingpotentials, a second thermionic valve having a control electrode, acircuit including the impedance of said second thermionic Valveconnected between the grid and cathode of said thermionic repeater, aconnection between the control electrode of said second valve and saidsource of modulating potentials for impressing said modulatingpotentials on said second valve to modulate said repeater, and aconnection between said source of modulating potentials and the controlelectrode of said first thermionic Valve to control the same inaccordance with said modulating potentials to overcome the normal biasapplied to said repeater.

FREDERICK CHARLES LUNNON. HAROLD JOSEPH HUGHES WASSELL.

